Not happy not knowing with which of the chairs to pick, I felt the need for a quick play with Excel.

Mars' average temperature today is around 210K. All things being equal, a 70%-as-bright Sun only knocks about 18K off that - a figure which I find a little suprising in terms of how relatively undramatic a change that is.

I can dial in a current-Earth-like atmosphere for this early Mars, and almost regain that 18K. Still cold and dry!

But consider a short-lived, even thicker atmosphere, one loaded with CO2 and water vapour pumped out by those gigantic volcanoes, and then the temperature starts to rise markedly. Is this the way to a warm, wet Mars? Sadly I don't think so, since this atmosphere comes at a price: it's going to seriously raise the albedo, dropping the temperature.

It's hard work to reach an Earth-like 288K.

Indeed, only towards the limits of Venusianess does the planet become balmy, which I think is extremely unlikely to have occured or extremely short-lived if it ever did.

So - for most of Mars' history - it's definitely the cold, dry chair for me.

If Mars ever had an Earth-density atmosphere, though, it would have had far more CO2 and thus a stronger greenhouse effect. Earth's atmosphere is unstable, maintained that way by life. Before plants, Earth had much more CO2 and less O2.

If Mars ever had an Earth-density atmosphere, though, it would have had far more CO2 and thus a stronger greenhouse effect. Earth's atmosphere is unstable, maintained that way by life. Before plants, Earth had much more CO2 and less O2.

I believe the gist of AndyG's post is that even thick CO2 atmosphere doesn't raise temperature enough to make liquid ocean possible.

The pressure required to maintain the liquid state doesn't have to depend on a thick atmosphere; it can come from the weight of 'sea ice'. Most of the water oceans in the solar system have such caps, but I think any ancient martian ocean would have had a much thinner cap than that of Europa or the other galileans, more like 'snowball Earth'.

The pressure required to maintain the liquid state doesn't have to depend on a thick atmosphere; it can come from the weight of 'sea ice'. Most of the water oceans in the solar system have such caps, but I think any ancient martian ocean would have had a much thinner cap than that of Europa or the other galileans, more like 'snowball Earth'.

Sea ice would be unstable in current Mars conditions - it would sublimate.

With thicker atmosphere in the past, it looks plausible. So my mental picture needs to be corrected: ancient Mars ocean, where it was liquid, was covered by permanent thick ice cover (many meters). Over the eons, atmosphere was lost, water was partly sublimating and escaping to space and to colder, higher latitudes, and partly "escaping" to permafrost and deep underground aquifers. Surface of ex-frozen-ocean was gradually covered by dust.

Bear in mind that this description of "cold Mars" is too simplistic. In 3 billion years, a lot of interesting events happened. Largish impacts. Changing tilt of the rotational axis. And first of all, volcanic eruptions and lava floods are prime candidates to make a lot of water liquid, at least for some geologically short time, but enough to cause catastrophic floods, carve river valleys, etc...

Are you taking into account the salinity of the sea, and also more methane than is currently observed?

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"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau

Are you taking into account the salinity of the sea, and also more methane than is currently observed?

Consider this: on Mars equator, average temperatures are below -40C. This is at least 20C colder than on North Pole of the planet Earth. The Earth has thick atmosphere with some CO2, with traces of methane, and its ocean is saline, and yet, ice at North Pole is 2-3 meters thick.

The hypersaline lakes in the McMurdo Dry valleys remain ice free most of the year, and without tectonics the martian ocean could have been shallower than on Earth.

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"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau

"Lake Vida is one of the largest lakes in the McMurdo Dry Valley region and is a closed-basin endorheic lake. The top waters of Lake Vida are frozen year-round to a depth of at least 19 m forming an ice-seal over briny waters that are seven times as saline as seawater. ... The high salinity allow for the lake bottom waters to remain liquid at an average yearly water temperature of -10°C."

Which raises an interesting question. Could ice-covered lakes (like Lake Vostok, for example) still be present on Mars? How would one search for them? The ice would likely be mantled with dust and lag deposits. They would most likely occur at mid latitudes where ice is more stable. And they would likely be Amazonian in age (?). It seems if the zone of liquid water were shallow enough that MARSIS or SHARAD could detect the ice-water boundary. All questions I don't know the answer to. Does anyone know if anyone has looked into this?

Scientists should be able to see the top of a liquid zone somewhere in the upper 2-3 kilometers (1-2 miles) fairly easily, and may be able to go down to 5 km (about 3 miles) or more.

--------------------

"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau

Which raises an interesting question. Could ice-covered lakes (like Lake Vostok, for example) still be present on Mars? How would one search for them? The ice would likely be mantled with dust and lag deposits. They would most likely occur at mid latitudes where ice is more stable. And they would likely be Amazonian in age (?). It seems if the zone of liquid water were shallow enough that MARSIS or SHARAD could detect the ice-water boundary. All questions I don't know the answer to. Does anyone know if anyone has looked into this?

Both the MARSIS and SHARAD folks have looked for stuff like this, as well as evidence of subsurface liquid water aquifers. So far they haven't found anything conclusive...they've found lots of subsurface ice in various places around the planet (i.e. the aprons in the Deuteronilus Mensae), but nothing suggesting liquid water.

Both the MARSIS and SHARAD folks have looked for stuff like this, as well as evidence of subsurface liquid water aquifers. So far they haven't found anything conclusive...they've found lots of subsurface ice in various places around the planet (i.e. the aprons in the Deuteronilus Mensae), but nothing suggesting liquid water.

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